You mentioned you didn't want the ammonia to spike.... pH and ammonia are unrelated, as turtle said pH is how acidic or basic your water is (lower than 7 acidic, higher than 7 basic). Ammonia is a product of the breakdown of fish food, fish waste, decaying plant material, etc.

Also, I don't think it was mentioned yet, but a stable pH is more important than a specific pH.... unless it's really low or really high. Therefore, if you're constantly having to adjust the tap water's pH with chemicals, there will be much fluctuation of the pH than if you just used the tap water without adjusting the pH.

It's also just plain less of a PITA to use the tap water without having to alter the pH all the time! Spend your hard earned $$ on gifts for your betta instead of pH chemicals! :)